High performance graphics devices are becoming increasingly important in today's computer systems. Computer system manufacturers commonly install graphics accelerators on add-in cards that plug into connectors that provide connection to a graphics bus such as an accelerated graphics port (AGP) bus. Graphics accelerators are also commonly installed by soldering the component directly onto computer system motherboards. The add-in card method provides the computer system manufacturer and the end user a means for installing a newer model graphics accelerator. One can simply remove the old add-in card and install a new one. The add-in card method has a drawback in that the add-in card method is more expensive than the solder-down method. The add-in card also uses more space than a device soldered directly onto the motherboard.
The solder-down method provides a cost savings over the add-in card method, but has the disadvantage of not providing a method for upgrading the graphics accelerator. If a computer system manufacturer wishes to change to a different graphics accelerator, the computer system manufacturer must redesign the motherboard. Further, the AGP bus is meant to be a point-to-point interconnect between a system memory controller and a single graphics device, and prior computer systems that utilize an AGP bus are not able to provide an upgrade option when a graphics device is already soldered onto the motherboard.
An AGP graphics device may have the ability to operate in one or more of several data transfer modes, including 2X mode and 4X mode. The 2X mode allows the transfer of 8 bytes of data per clock cycle using a pair of data strobe signals. The 4X mode allows the transfer of 16 bytes per clock cycle using 2 pairs of data strobe signals.
A prior 2X mode AGP graphics device normally includes input/output buffers for two address/data strobe signals (AD_STB0 and AD_STB1) and an input buffer for a sideband strobe signal (SB_STB). An AGP bus that allows 4X mode operation provides compliments of the above strobe signals (AD_STB0#, AD_STB1#, and SB_STB#) in addition to the above strobe signals. When the prior 2X mode AGP graphics device is installed on the AGP bus capable of 4X mode operation, the strobe compliment signals in prior systems are not connected at the prior 2X mode AGP graphics device. If an upgrade 4X mode AGP graphics device is installed on the 4X mode capable AGP bus in parallel with the prior 2X mode AGP device, a situation is created where the AD_STB0, AD_STB1, and SB_STB signals have three capacitive loads (one at a memory controller, one at the prior 2X mode AGP graphics device, and one at the upgrade 4X mode AGP graphics device) and the AD_STB0#, AD_STB1#, and SB_STB# signals have two capacitive loads (one at the memory controller and one at the upgrade 4X mode AGP graphics device). This difference in capacitive loading between the strobe signals and their compliments would have a negative impact on graphics bus timing relationships, and therefor a negative impact on system reliability.